Make StockEvent a base class for Delivery and StockTaking

This helps to share code between the two entities and allows easier
extensions in the future.

app/models/stock_taking.rb

@@ -1,10 +1,4 @@
-class StockTaking < ApplicationRecord
-
-  has_many :stock_changes, :dependent => :destroy
-  has_many :stock_articles, :through => :stock_changes
-
-  validates_presence_of :date
-
+class StockTaking < StockEvent
   def stock_change_attributes=(stock_change_attributes)
     for attributes in stock_change_attributes
       stock_changes.build(attributes) unless attributes[:quantity].to_i == 0